Fluid tight doors often require extensive adjustments and readjustments in order to continuously maintain a fluid tight closure in a dynamic environment. Fundamental problems are the stiffness, or non-resiliency, of the existing door gasket; and the flexing and working of a ship's hull at sea, or on an aircraft in flight. This twisting and flexing often causes warping, which results in bulkhead or airframe knife edge bending. Knife edge flexing and bending causes uneven seating between the knife edge and the gasket. The result is the loss of a fluid tight seal when the door is closed and sealed.
Where the knife edge is secured to the fixed door frame, the gasket is secured to the movable door. Alternately, the knife edge may be secured to the movable door, and the gasket secured to the fixed door frame.
In order to overcome the warpage, and thereby seal the door, existing elastomeric gaskets require large handle latching torques to assure line-to-line contact between the gasket and knife edge. Existing gaskets rely upon a large mechanical compression force to firmly press the knife edge against the gasket to achieve a fluid tight seal.
There are three basic types of seals now in use: mechanical compression seals; flap seals; and self energizing seals.
Mechanical compression seals seal a fluid by forcing a blunt knife edge into a flat elastomeric gasket. The knife edge is usually made of metal and is typically attached to the fixed member, while the elastomeric gasket is generally secured to the movable member. Mechanical compression seals prevent fluid leakage across the seal from either direction. Fluid leakage will occur when the differential pressure across the seal exceeds the mechanical compression force used to press the knife edge against the gasket.
Flap seals comprise an elastomeric flap, generally attached to the movable member, which presses against the solid surface of the fixed member. Differential pressure across the seal causes the flap to press against the fixed member to prevent fluid leakage. Flap seals act as check valves, sealing the flap valve when the differential pressure is acting to press the flap valve against the fixed member. Flap valves do not seal effectively against differential pressure acting to push the flap valve away from the fixed member. Two opposing flap valves may be used in opposing configurations to seal against differential pressure acting from either direction.
Self-energizing seals, use differential fluid pressure across the seal to press the gasket against both the fixed and movable members. The elastomeric gasket is placed in compression between the fixed and movable members to form a pre-compression force, which remains a constant value that does not change as the differential pressure increases across the seal. As fluid pressure increases on the pressure side of the seal, the fluid pressure is transmitted throughout the elastomeric gasket, causing the attempted expansion of the elastomeric gasket against the inner and outer flat surfaces of the fixed and movable members. Since the inner and outer flat surfaces cannot move, the contact pressure increases in proportion to the fluid pressure on the pressure side of the gasket. The sum of the pre-compression force is added to the differential pressure on the seal, to assure a fluid tight seal.
The self-energizing seal is fluid tight for any value of differential pressure, and failure can only occur when the differential pressure exceeds the designed stress limits of the seal.
An O-ring utilizes this self-energizing principle in order to impart a sealing force when used to hydraulically seal a shaft. A pre-compression force is exerted between the fixed member, the O-ring and the movable member. Differential pressure is transmitted throughout the O-ring which forces the O-ring against one wall of the slot. Attempted expansion of the O-ring provides increased compression pressure between the inner and outer surfaces, resulting in a self-energizing fluid seal, utilizing both precompression and differential pressure forces.
The following prior art is representative of fluid tight seals used to seal a hatch or bulkhead:
U.S. Pat. No. 4,891,910 issued Jan. 9, 1990, entitled "Apparatus for sealing a door", discloses an elastomeric flat seal which seals against differential pressure by means of mechanical compression against a flat face.
U.S. Pat. No. 4,685,249, issued Aug. 11, 1987, entitled "Industrial Air Filter Door with Standard Handle-Dog Actuation", discloses a resilient oval cross section seal, which seals against differential pressure by mechanical compression forces acting between two flat surfaces.
U.S. Pat. No. 4,545,764 issued, Oct. 8, 1985, entitled "Rotary Kiln Assemblies, Method of Changing Seal Arrangements and Seal Arrangements for use in a Rotary Kiln Assembly", discloses two flap type seals mounted in an opposing configuration to seal against limited differential pressure acting from either side of the seal.
U.S. Pat. No. 4,523,407, issued Jun. 18, 1985, entitled "hatch Cover", discloses a self-energizing, hollow cavity, O-ring seal which is encased within a retainer. This O-ring functions as a face seal and is self-energizing under differential pressure. No claim or disclosure is provided, relating to the sealing of the ends of the cavity in order to build up an internal equalizing pressure within the cavity of the O-ring, when the O-ring is under compression.
U.S. Pat. No. 2,844,188 issued Jul. 22, 1958, entitled Water Tight Hatch Cover Arrangement, discloses a rectangular seal with an internal cavity, which is retained by a bead and biased from the side as shown in FIGS. 1 and 2. A projecting member engages the gasket to seal the hatch cover.
U.S. Pat. No. 3,043,257 issued Jul. 10, 1962, entitled FLUSH DECK HATCH COVER discloses a flush deck hatch cover having a resilient gasket with an internal aperture which is compressed against a flat stop to seal the hatch.
U.S. Pat. No. 2,964,304 issued Jul. 26, 1960, entitled HATCH COVER ASSEMBLY, discloses a hatch cover assembly having a central seal formed by resilient members each having an internal cavity, which abut each other to seal the hatch. See FIG. 5.